Ethynyl-substituted benzosiloxaboroles: the role of C(π)⋯B interactions in their crystal packing and use in Cu(i)-catalyzed 1,3-dipolar cycloaddition

The synthesis and characterization of two novel 6-ethynyl-7-halogen substituted benzosiloxaboroles (Hal = F, Cl) is reported. The crystal structures of these compounds show a unique type of supramolecular assembly dictated by distinctive C(π)⋯B interactions resulting in the formation of columnar networks involving alternating ethynyl groups and boron atoms. The QTAIM, NBO and NCI analyses were performed in order to obtain a deeper quantitative insight into the nature of these interactions including energy and charge density distribution. The fluoro derivative 1c was used as a starting material in Cu-catalyzed 1,3-dipolar cycloaddition reactions with substituted benzenesulfonyl azides giving rise to benzosiloxaboroles with pendant 1-(arylsulfonyl)-1,2,3-triazole-4-yl functionalities or analogous ionic species, i.e., 1,2,3-triazolium arylsulfonates. Screening of antimicrobial activity of obtained derivatives against a wide selection of Gram-positive and Gram-negative bacteria as well as fungi strains was performed and the obtained results were compared with the data obtained previously for related benzosiloxaborole derivatives.


Introduction
A specic class of cyclic boron hemiesters called benzoxaboroles have been known for 70 years 1 but they have attracted increased attention only during the last 15 years in the eld of medicinal chemistry. 2Extensive studies revealed that functionalized benzoxaboroles are promising small-molecule therapeutic agents possessing antibacterial, antifungal, anti-inammatory and anticancer activity. 3For example, uorinated benzoxaborole called tavaborole (trade name Kerydin) exhibits high antifungal activity and was commercialized to treat onychomycosisa fungal infection of the nail and nail bed. 4 5-(4-Cyanophenoxy)benzoxaborole called crisaborole (trade name Eucrisa) was approved for the treatment of mild-tomoderate atopic dermatitis (eczema). 5The activity of benzoxaboroles relies primarily on the binding of the Lewis acid boron centre to the biological target via the formation of a strong covalent bond.The mechanism of action of benzoxaboroles relies on their physicochemical specicity based on the enhanced character of the boron atom.The examples of approved benzoxaborole therapeutic agents as well as other derivatives which are currently under investigation demonstrate that the mode of bioactivity depends on structural modication of the benzoxaborole core.In 2015, our group proposed benzosiloxaboroles as silicon congeners of benzoxaboroles and demonstrated high antifungal activity of simple uorinated derivatives. 6Further research revealed the potency of selected derivatives including oxaborole-benzosiloxaborole hybrid II as KPC b-lactamase inhibitors. 7Recently, we found that the antibacterial activity of benzosiloxaboroles involves a different mechanism than for related benzoxaboroles pointing to the importance of the introduced structural change. 8In fact, replacement of the CH 2 with the SiMe 2 group in the oxaborole ring results in increased Lewis acidity and lipophilicity which may be responsible for specic biological activity.Recently, we have also studied crystal structures of functionalized benzosiloxaboroles showing that there is a strong effect of substitution pattern on their supramolecular assembly, mostly manifested by formation of various hydrogen-bonded motifs. 9Those results revealed that benzosiloxaboroles are interesting also from the perspective of crystal engineering.
In medicinal chemistry, the conjugation a core of a molecule with different pharmacophores is one of key synthetic concepts as resulting hybrid structures may show higher affinity to a biological target.Herein, we turned our attention to triazole scaffold as it is present in many compounds showing diverse biological properties.For example, vicinal diaryl triazoles were RSC Advances PAPER found to be promising tubulin polymerization inhibitors, COX-2 inhibitors, and CB1 receptor antagonists. 10Essramycin (Scheme 2), is (1,2,4-triazolo)pyrimidine derivative active against several Gram-positive and Gram-negative bacteria such as Bacillus subtilis, Staphylococcus aureus, Micrococcus luteus, E. coli, and Pseudomonas aeruginosa. 11Triazole-containing compounds are widely used as topical and systemic antifungal agents.Fluconazole, the rst-generation 1,2,4-triazole antifungal agent, is used for fungal infections, including candidiasis, blastomycosis, cryptococcosis, histoplasmosis, coccidioidomycosis, dermatophyteosis and pityriasis versicolor. 121,2,3-Triazole moiety is a part of tazobactaminhibitor of b-lactamases from class A according to Ambler classication (especially SHV, TEM and CTX-M enzymes). 13Tazobactam combined with the b-lactam antibiotic such as piperacillin is used to treat infections caused by Pseudomonas aeruginosa.Moreover, tazobactam with the new cephalosporin ceolozane is dedicated for treatment of the extended-spectrum b-lactamase (ESBL)-producing Enterobacterales and multidrugresistant (MDR) P. aeruginosa infections.13b Runamide is a new anti-epileptic drug used in combination with other medication and therapy to treat Lennox-Gastaut syndrome and various other seizure disorders. 14In the context of this work, a recent synthesis of a novel boronic acid (MB076) with a pendant 1,2,3-triazole-4-carboxylic acid moiety as a highly effective inhibitor of class C Acinetobacter-derived cephalosporinases is interesting. 156-Substituted triazolyl benzoxaboroles were very recently identied as selective carbonic anhydrase inhibitors. 16s a continuation of our studies on functionalized benzosiloxaboroles, we present herein the synthesis and in-depth structural characterization of ethynyl-substitited derivatives which highlights the effect of the p-hole on the boron atom on crystal packing.These relatively simple compounds were converted to derivatives bearing 1-arylsulfonyl-1,2,3-triazole-4-yl moiety.Such a structural design was inspired by our previous results showing that arylsulfonamido 17  methicillin-sensitive Staphylococcus aureus (MSSA), methicillinresistant S. aureus (MRSA), Enterococcus faecalis and Enterococcus faecium.Based on the presented literature background, [10][11][12][13][14][15][16] we assumed that the introduction of 1,2,3-triazole spacer between arylsulfonyl group and benzosiloxaborole might have further positive impact on the antimicrobial potency.

Results and discussion
2.1 Synthesis and crystal structure of 6-ethynyl-7-halogen substituted benzosiloxaboroles 1c and 2c In a three-step procedure, 4-bromo-2-uoro-1-iodobenzene 1 (Scheme 3) was converted to the respective TMS-protected ethynyl derivative 1a via Sonogashira coupling as reported previously 19 followed by deprotonation with LDA at −78 °C and trapping of corresponding aryllithium intermediates with Me 2 Si(H)Cl to give arylsilane 1b in high yield.The lithiation occurred regioselectively at the 3-position anked by two halogens which is activated by a strong cumulated ortho-acidifying effect of those two substituents. 20Compound 1b was subjected to Br/Li exchange with t-BuLi in Et 2 O at −100 °C followed by trapping with B(OMe) 3 .The hydrolysis effected with 1 M NaOH/ H 2 O mixture resulted in a cleavage of the Si-H bond which occurs rapidly due to ortho-assistance of the anionic boronate group. 21The strongly alkaline conditions result also in removal of the trimethylsilyl group leading directly to the targeted derivative 1c which is the rst example of the ethynylsubstituted benzosiloxaborole (Scheme 3).7-Chloro-6-ethynyl derivative 2c was obtained using a similar three-step protocol starting with 4-bromo-2-chloro-1-iodobenzene 2. The overall yields of 1c and 2c were 77 and 62%, respectively.Both products were isolated as powders well soluble in most organic solvents.The 1 H and 13 C NMR spectra of 1c and 2c are in agreement with their structures whilst 11 B NMR resonances at ca. 30 ppm are typical of other benzosiloxaboroles. 6It is worth noting that according to our best knowledge there are no published examples of ethynyl-substituted benzoxaboroles, i.e., direct analogues of 1c and 2c.

Crystal structures 1c and 2c: the role of C(p)/B interactions
The geometry of benzosiloxaborole core in 1c is essentially the same as in related compounds reported by us previously. 9The supramolecular assembly of 1c shows centrosymmetric hydrogen-bonded (HB) dimers (O/O distance of 2.784(1) Å, motif D1, Fig. 1a) typical of boronic acid derivatives 22 including most of benzoxaboroles and many benzosiloxaboroles. 6,22owever, the molecules within a dimer are not coplanar but they are signicantly displaced one to another so the angle between mean planes dened by the four O atoms of the dimeric motif and the atoms of the siloxaborole ring is 26.7°.Such geometrical deformations are also reected in decreased interaction energy between molecules.According to DFT calculation performed at M06-2X/6-311++G(d,p) level of theory, the D1 dimer interaction energy in 1c is equal to 53.4 kJ mol −1 , i.e., lower compared to typical HB dimers in benzoxa-and benzosiloxaboroles (∼58 kJ mol −1 ).In addition, boron atoms show characteristic interactions with the p-density of two ethynyl groups of neighboured molecules (motifs D2a and D2b, Fig. 1c) characterized by respective intermolecular B/ C(terminal) contacts of 3.394(2) (motif D2a) and 3.626(2) Å (motif D2b).The C(p)/B interactions, which can be considered as a type of p-hole triel bonds, 23 are well visible on the Hirshfeld surfaces mapped over the d norm property as shown in Fig. 2a.Specically, the two major red spots are associated to O-H/O hydrogen bonds, while the two other red spots visible above the boron atom and ethynyl group correspond to C(p)/B interactions.Overall, the propagation of D2 dimeric motifs along the [100] direction leads to the formation of a 1D columnar assembly (Fig. 1c).The important contribution from p-stacking interaction between parallel aromatic rings should be also encountered thus giving rise to quite signicant D2a and D2b dimer interaction energies of 34.5 and 37.3 kJ mol −1 , respectively (Table 1).
In order to get deeper insight into the nature of C(p)/B interaction we have performed natural bond orbital (NBO) analysis and, in parallel, topological analysis of electron density within quantum theory of atoms in molecules (QTAIM) approach.The energetic contributions of intermolecular donor-acceptor interactions was estimated by 2nd-order perturbation theory (Table S1, ESI †).It comes out that the formation of C(p)/B interaction mostly originates from electron donation from ethynyl p CC orbital to empty p B orbital (orbital interaction energy E = 5.1 kJ mol −1 in D2a and E = 2.6 kJ mol −1 in D2b dimers, Fig. 2b).It is supported by the backdonation from phenyl p-electron density (represented by localized p CC orbital) to antibonding p* CC of ethynyl group (E z 1.0 kJ mol −1 ).In addition, the non-covalent interaction index (NCI) was calculated for 2Da dimer.It revealed the typical pattern of p-stacking interactions reected by the green area covering almost the whole interface between two adjacent aromatic rings (Fig. 2c).The reduced gradient density (RGD) isosurface is extended on the regions corresponding to the C(p)/B contact conrming the weak attractive character of this interaction.The QTAIM analysis revealed the appearance of bond critical point (BCP) located in-between the ethynyl group and boron atom with the electron density (r) of 0.036 e Å −3 and negative Laplacian (V 2 r) of 0.34 e Å −5 .The amount of electron density at BCP is slightly lower compared to the value found at BCP in model BCl 3 -ethynyl complex (r = 0.045 e Å −3 ) studied by Grabowski, 24 and lies in the range of typical density values for two stacking aromatic rings (0.03-0.07 e Å −3 ). 25 At this point, it should be noted that the bond path does not directly connect the ethynyl triple bond and boron atom but it ends at BCP of the B-C bond.Similar effect was already described for C]C(p)/B interaction in the crystal structure of (E)-(4-phenylbut-1-en-1-yl) boronic acid. 26The bond path might not be revealed from QTAIM analysis despite an attractive character of the C(p)/B interaction.Moreover, the presence of electron rich oxygen atoms at the boron atom may hinder the formation of the mentioned bond path.
The crystallization of 1c in acetonitrile afforded monosolvate 1c$MeCN.Unlike crystal structures of most benzoxaboroles Compound 2c forms non-solvated crystals both from CHCl 3 and MeCN solution.It is isostructural with 1c which means that the replacement of uorine with chlorine has a marginal effect on the supramolecular assembly.As for 1c, centrosymmetric dimers are formed (O/O) distance of 2.793(2) Å, i.e., by only 0.005 Å longer compared to (1c) and molecules within a dimer are signicantly displaced one to another at the same angle of 25.9°as for 1c.Owing to the longer O/O distance, the interaction energy between molecules is slightly lower compared to 1c (50.7 kJ mol −1 ).The C(p)/B interactions are characterized by respective intermolecular B/C(terminal) contacts of 3.362(2) and 3.640(2) Å, i.e., distance alternation is slightly smaller than for 1c.This resulted in increased intermolecular interaction energy for D2a (41.4 kJ mol −1 ) and D2b (42.2 kJ mol −1 ) dimers.Finally, C(p)/B orbital interaction energies and AIM parameters at corresponding BCPs are comparable for all studied systems (Tables S1.3 and S1.4,ESI †).

The use of 1c in Cu(I)-catalyzed 1,3-dipolar cycloaddition
Compound 1c was utilized in Cu(I)-catalyzed azide-alkyne 1,3dipolar cycloaddition (CuAAC) reactions with selected organic azides with a special emphasis on arylsulfonyl azides (easily accessible from the nucleophilic substitution of respective chlorides with NaN 3 in acetone 27 ).Previously, thermally induced azide-alkyne cycloaddition was reported for the preparation of a some boronated 1,2,3-triazoles. 28There are only a few reports on arylboronic-triazole conjugates up-to-date.For the rst time, such compounds synthesized via CuAAC were reported by Hall et al. 29 Kumar et al. synthesized a library of 1H-1,2,3-triazole-tethered 4-aminoquinoline-benzoxaborole hybrids and aryl-substituted benzoxaborole analogues.Obtained products were screened for their anti-plasmodial efficacy against both chloroquine-susceptibility 3D7 and chloroquineresistant W2 strains of P. falciparum. 30The Passerini three component selective synthesis of benzoxaboroles with pendant tetrazole substituents shows also some resemblance to the present work due to the use of boronic acid and azide precursors. 31The standard CuSO 4 /sodium ascorbate catalytic system 32 proved not effective in our CuAAC-based protocol utilizing 1c and various arylsulfonyl azides as starting materials.In contrast, Cu(I) thiophene-2-carboxylate (CuTC, 0.1 equiv.)proved more effective as a catalyst while the reactions were conducted in a biphasic water/toluene mixture (1 : 1). 33owever, the use of simple benzenesulfonyl azide did not afford the expected cycloaddition product.Instead, the respective 4-(7-  a The intermolecular distance between heavy atoms.b Interaction energies calculated at M06-2X/6-311++G(d,p) level of theory, positions of non-hydrogen atoms were retained from the crystal structure, and positions of hydrogen atoms were fully optimized.
Overall, the cycloaddition reaction proceeds smoothly but it is difficult to control the subsequent potential abstraction of arylsulfonyl moiety.All products were characterized by 1 H, 13 C and 19 F NMR spectroscopy as well as HRMS.In the NMR spectra of 4a-4c the signal of the proton at the 5-position of 1,2,3-triazole ring at ca. 8.8-9.1 ppm appears always as a doublet with a coupling constant of ca.3.5 Hz which may be attributed the existence of a through-space 1 H- 19 F coupling with the uorine atom.In addition, compound 4d bearing the pendant 2-uorophenylsulfonyl moiety was characterized by single crystal Xdiffraction (Fig. 3a) as a monosolvate 4d CHCl 3 .It should be noted that we were unable to obtain pure bulk 4d and the crystal was grown from the mixture containing mainly a respective ionic species analogous to 3a-3b.In the molecule 4d the triazole ring is essentially coplanar with the benzosiloxaborole core (the interplanar angle of benzene and triazole ring is 10.5°) as that conformation apparently gains stabilization owing to the short intramolecular C10(H)/F1 contact of 2.37 Å which is in agreement with 1 H NMR data.Compound 4d also forms dimers but the angle of displacement of molecules within a dimer (dened above for 1c) is only 10.3°(Fig.4a).The packing also shows stacking of the adjacent molecules characterized by interactions of the boron atom with the p-density of the triazole rings.The distance between the B atom and triazole ring centroid is 3.61 Å.The partially disordered CHCl 3 molecules are closely paired 34 in the crystal structure cavities and show Hbonding interactions with triazole rings as the C19(H)/N3 distance is relatively short (3.193Å).
The geometry of the cationic part of compound 3b is similar to that of the analogous fragment in 4d (Fig. 3b).Unlike 4d, the cations do not assemble through the formation of centrosymmetric dimeric motifs involving B(O)OH groups.Instead, two types of charge-assisted NH/O hydrogen bonds (d N/O = 2.684 Å, d N/O = 2.669 Å) link cationic triazolium moieties with two neighbored arylsulfonate anions (Fig. 4b).This is complemented by another hydrogen bond formed between the BOH Scheme 4 Application of Cu(I)-catalyzed azide-alkyne 1,3-dipolar cycloaddition for the synthesis of 1,2,3-triazole-4-yl functionalized benzosiloxaboroles 3a-3c and 4a-4c.group and the third oxygen atom of the sulfonate group (d O/O = 2.882 Å).Overall, this results in a layer network located symmetrically with respect to the (100) plane.The layers interact by means of weak C-H/Cl interactions of methyl groups with chloro substituents.In addition, we attempted to grow single crystals of the neutral compound 3c.However, we obtained invariably an amorphous material which can be attributed to dynamic equilibria between various forms of 3c, presumably involving proton exchange at the triazole ring and/ or aggregation processes involving formation of N-B coordination bonds.This is indicated by strongly broadened signals in the 1 H NMR spectrum of 3c (Fig. S3.17 †).
We have also studied the reactions of 1c with functionalized alkyl azides XCH 2 CH 2 N 3 (X = OH, NHSO 2 Ph).Since CuTC was not effective as a catalyst in both cases, we decided to use the classical system CuSO 4 /sodium ascorbate in H 2 O/THF mixture.This approach did not result in expected benzosiloxaboroles as cycloaddition reactions were accompanied by extensive protodeboronation, presumably catalyzed by Cu 2+ cations.Thus, the respective structurally extended 1,3-diaryl-1,1,3,3-tetramethyldisiloxanes 5a-5b were isolated in reasonable yields as isolable products of a dehydrative condensation of initially formed aryldimethylsilanols (Scheme 5).This was conrmed for 5b by X-ray crystallography showing a specic curled conformation of the molecule stabilized through two intramolecular NH/N hydrogen bonds (d N/N = 2.922 Å, d N/N = 2.929 Å) linking sulfonamide groups and triazole rings (Fig. 5).In addition, p-p stacking interactions of internal benzene and triazole rings play also some role (in the former case, it would be probably more precise to invoke mutual C-F dipole-dipole or C-F/p interactions).

Antimicrobial activity
Taking into account the direct antimicrobial activity of various benzosiloxaborole derivatives demonstrated previously 6,7,17,18,35 in the current research we determined the activity of the newly synthesized compounds against a wide panel of Gram-positive, Gram-negative bacteria and yeasts strains.All obtained results were summarized in Tables S2.   2 and S2.1 †).Among these active compounds, derivatives 2c and 3c were highly active against standard staphylococci, including MRSA, with MIC 12.5 mg L −1 .Despite the approval of new drugs and actions taken around the world to limit the spread of drug-resistant strains, MRSA strains are still one of the most common pathogens causing nosocomial infections.MRSA strains have been entered on the WHO list of the most dangerous pathogens for humans. 36It is worth emphasizing that MRSA strains are resistant to almost all blactams (except cearoline and ceobiprole) and are oen resistant to many other groups of antibiotics as macrolides, tetracyclines, aminoglycosides, and uoroquinolones. 37,38owever, the most promising compounds 2c and 3c, showed from 6-to 12-fold less activity than linezolid as a reference agent.Linezolid is a representative of a relatively new group of drugs active against cocci, including MRSA strains. 38In our previous publication, we reported that among benzenesulfonato benzosiloxaborole derivatives display the highest potency (MICs 0.39-3.12mg L −1 ) against Staphylococcus spp. 18However, expanding the structure of compounds tested in this study with arylsulfonyl group resulted in a reduction in activity against staphylococci and enterococci (from 2-to 4-fold reduction in the MIC values of compounds 3a, 3b, 4b, and 4c, compared to 3c) (Table 2).It seems that this can be attributed to the facile abstraction of the arylsufonyl group under aqueous conditions resulting in the formation of respective 1,2,3-triazolium arylsulfonates where only the cationic part comprising the Scheme 5 Formation of functionalized 1,3-diaryl-1,1,3,3-tetramethyldisiloxanes 5a-5b.
Fig. 5 The molecular structure of 5b viewed from two different perspectives.a The highest activity against Gram-positive bacteria indicated by the low MIC values (#12.5 mg L −1 ) is shown in boldface.(-) the inhibition zone was not observed in the disc-diffusion method.The diameter of the paper discs was 9 mm.b Only the MBC values #400 mg L −1 are presented.c The Eagle effect 39 was observed during the determination of the MBC value of the same tested agents against Staphylococcus spp.strains.The Eagle effect is shown in the italic face.d LIN, linezolid was used as a reference agent active against Gram-positive bacteria.The diameter of a commercial disc containing 0.03 mg of linezolid was 6 mm; the MIC of linezolid was determined according to the CLSI recommendations. 40iloxaborole ring exhibits antibacterial acitivity.For most compounds, the minimal bactericidal concentration (MBC) values were high (>400 mg L −1 ).Notably, the so-called Eagle effect (also known as the paradoxical growth reported for various antibiotics) 39 was observed in the case of S. aureus ATCC 6538P (for 1c, 2c and 3c) and S. epidermidis ATCC 12228 (for 1c and 3c).It implies occurrence of two MBC values as determined by us previously for other substitued benzosiloxaboroles. 17,18he rst MBC values were 2-fold higher than the MIC values.Furthermore, a progressive increase in the number of surviving bacteria was observed at higher concentrations followed by a subsequent decrease.If the bacterial population was reduced again to the MBC threshold, a second MBC value (at 200 $ 400 mg L −1 ) could be determined.Only 1c and 2c displayed weak activity against Gram-negative rods with MICs ranging from 200 to >400 mg L −1 (Table S2.2 †).Considering that resistance of Gram-negative rods is frequently associated with efflux pump activity, 41 we also determined the MICs of newly compounds in the presence of efflux pump inhibitor, i.e., phenylalanine-arginine-b-naphthylamide (PAbN). 42In the case of Escherichia coli, Klebsiella pneumoniae, Serratia marcescens and Stenotrophomonas maltophilis strains the MICs of compounds 1c, 2c and 3c are reduced at least 4-fold in the presence of PAbN.This means that these compounds are actively removed by efflux pumps of bacterial cells.
Considering previously reported antifungal activity of some benzosiloxaboroles (with MICs ranging from 0.78-12.5 mg L −1 ) 6,7 we investigated the activity of new derivatives against 7 standard yeast strains.The moderate antifungal activity was observed only for 1c and 2c with MICs ranging from 12.5 to 400 mg L −1 (Table S2.3 †).

Conclusions
In conclusion, a convenient protocol for the synthesis of halogenated benzosiloxaboroles 1c and 2c comprising reactive ethynyl substituents was elaborated.It should be noted that analogous functionalized benzoxaboroles are still unknown which strengthens the importance of presented results.Intriguingly, C(p)/B interactions seem to play a decivise in role in supramolecular organization of these compounds which is conrmed by the fact that it is retained in the crystal structure of the solvate 1c MeCN.This observation indicates a high potential of the ethynyl group in crystal engineering of boracyclic compounds.CuAAC reaction of 1c using arylsulfonyl azides gave rise to respective products 4a-4c bearing 1-(arylsulfonyl)-1H-1,2,3-triazol-4-yl functionalities.However, compounds of this type are rather prone to hydrolysis as evidenced by the isolation of 1,2,3-triazolium arylsulfonate salts 3a-3b and the neutral benzosiloxaborole 3c with attached 1,2,3triazol-4-yl moiety.In turn, the analogous cycloaddition reactions with functionalized alkyl azides XCH 2 CH 2 N 3 (X = OH, NHSO 2 Ph) were accompanied by extensive protodeboronation and thus furnished respective 1,3-diaryl-1,1,3,3tetramethyldisiloxanes 5a-5b as isolable products.Structural studies of nal products were complemented by evaluation of antimicrobial actitivity which revealed that the ethynyl-substituted derivatives 1c and 2c are more potent antibacterial agents than structurally expanded 1,2,3-triazolyl derivatives.Based on previous ndings, we assumed initially that the presence of arylsulfonyl end groups could be benecial but it was denitely not the case.This might be rationalized by the tendency of these compounds to hydrolysis which can occur under physiological conditions.Further studies on the use of ethynyl-substituted benzosiloxaboroles as a platform for the construction of structurally extended systems including nucleoside conjugates are currently in progress and the results will be reported in due course.

General comments
Solvents used for reactions were dried by heating to reux with sodium/benzophenone and distilled under argon.Starting materials including halogenated benzenes, ethynyltrimethylsilane, Pd(PPh 3 ) 4 , CuI, chlorodimethylsilylane, alkyllithiums, diisopropylamine, trimethyl borate, sodium azide, CuTC, arylsulfonyl chlorides, as well as other reagents were used as received without further purication.In the 13 C NMR spectra, the resonances of boron-bound carbon atoms were not observed in some cases as a result of their broadening by a quadrupolar boron nucleus. 1 H and 13 C NMR chemical shis are given relative to TMS using residual solvent resonances. 11B and 19 F NMR chemical shis are given relative to BF 3 $Et 2 O and CFCl 3 , respectively.

Single crystal X-ray diffraction and structural analysis
Single crystals were grown by a solvent-evaporation method under air from the CHCl 3 or acetone solutions.Selected crystals were measured at low temperature (100 K) with the use of Oxford Cryosystems nitrogen gas-ow device.The crystal structures were established in a conventional way via X-ray data renement employing the Independent Atom Model (IAM).Data reduction and analysis were carried out with the CrysA-lisPro suites of programs. 44All structures were solved by direct methods using SHELXS-97 (ref.45) and rened using SHELXL-2016. 46All non-H atoms were rened anisotropically.All C-H atoms were placed in calculated positions with C-H distances of 0.95 Å and U iso (H) = 1.2U eq (C).The position H atoms of hydroxy groups were located from a difference electron density maps.
The O-H distances were xed to 0.84 Å with a standard deviation of 0.01 Å and the directionality of O-H was rened freely.The U iso parameter was set to 1.5U eq with respect to oxygen atoms.The crystal structure 1c contains large residual density peak (3.17 e Å −3 ), which cannot be reasonably explained by the mode.All-important crystallographic data including measurement, reduction, structure solution and renement details are placed in Table S1 47 Graphical plots of the Hirshfeld surfaces mapped with normalized contact distance (d norm ) use a red-white-blue color scale, where red indicates shorter contacts, white corresponds to contacts at the vdW distance, and blue is used for longer contacts.In addition, the Hirshfeld surfaces of all studied compounds were mapped with the values of electrostatic potential and fragment patch, the latter represents the fragment of the surface patches to adjacent molecules (Fig. S1.2 and S1.3, ESI †).
4.3.2Theoretical calculations.Theoretical calculations were performed at M06-2X[47]/6-311++G(d,p) 48,49 level of theory using Gaussian16 program. 50The dimeric interaction energies were calculated starting from the geometries extracted from the crystal structures followed by the optimisation of positions of hydrogen atoms (positions of non-hydrogen atoms were retained from the crystal structure).The interaction energy was calculated using the counterpoise procedure, which includes the correction for basis set superposition error (BSSE).The topological analysis of the electron density was prepared using QTAIM approach 51 and it was carried out with AIMAll package. 52he wavefunction was generated at M06-2X/6-311++G(d,p) level of theory.In the framework of this approach, bond baths (BP) and bond critical points (BCP) were identied.The noncovalent interaction index (NCI) analysis 53 was performed with AIMAll.The natural bond orbital (NBO) 54 analysis was calculated at the same level of theory.The localized orbitals were visualized using Avogadro soware.4.4.2Determination of antimicrobial activity.The direct antimicrobial activity against yeast, Gram-positive and Gramnegative bacterial strains was evaluated as previously described 17,18 by the disc-diffusion method and the MIC determination assays according to the EUCAST 56 and CLSI 40,57a recommendations.Bactericidal (MBC) activity was performed according to the CLSI recommendations.57b Each 0.02 mL bacterial culture samples from clear wells with the MIC and above the MIC values were transferred onto TSA agar plates (bioMerieux) and incubated for 24-48 h at 37 °C.The MFC was dened as the lowest concentration of a agent that kills 99.9% of the initial inoculum of yeast and was evaluated aer establishing the MIC values according to the EUCAST recommendation.56b Each 0.1 mL yeast culture samples from clear wells with the MIC and above the MIC values were transferred onto Sabouraud dextrose agar plates (bioMerieux) and incubated for 24-48 h at 30 °C.The following reference agents were utilized: uconazole (in the case of yeast), linezolid (for Grampositive bacteria), and nitrofurantoin (for Gram-negative rods).The new compounds were dissolved in DMSO (Sigma).In the disc-diffusion method, paper discs containing 0.4 mg of the tested new compounds per disc were used.The MIC and MBC/MFC values were determined up to 400 mg L −1 .
4.4.3Determination of MICs in the presence of PAbN.To determine the ability of the Gram-negative bacterial strains to remove new tested compounds by MDR efflux pumps, the MIC values of these agents, with or without the pump inhibitor, PAbN (20 mg L −1 ) (Sigma) were evaluated as previously described. 17,18To minimize the inuence of PAbN on the destabilization of bacterial cell covers, the tests were conducted in the presence of 1 mM MgSO 4 (Sigma). 58At least a 4-fold reduction in the MIC value aer the addition of PAbN was considered signicant. 17,18 Scheme 2 Biologically active compounds comprising 1,2,4-and 1,2,3-triazole rings including recent examples of organoboron compounds.
including the case of non-solvated 1c, in 1c$MeCN the BOH group adopts anti conformation.The O atom of this group serves as a hydrogen bond acceptor for the ethynyl H atom of a neighbored molecule (d O/H = 2.21 Å, d C/O = 3.155(1) Å, motif D5) which results in the formation of a chain running along the c axis (Fig. 1b).The chains are cross-linked through interactions with MeCN molecules.First, the H atom of the BOH group is involved in O-H/N hydrogen bond (d N/H = 2.03 Å, d O/N = 2.853(2) Å, motif D6).In addition, the methyl group of the solvent guest molecule is anchored to two molecules of 1c through weaker C-H/F (d C/F = 3.542(2) Å, motif D4), C-H/O (d C/O = 3.189(2) Å, motif D3) and C-H/C(p-ethynyl) (d C/C = 3.267(2) Å, as a part of motif D4) interactions.This results in a creation of a layer parallel to the (010) plane.The layers are generally assembled through p-stacking interactions but again characteristic intermolecular B/C(terminal) symmetrical contacts of 3.528(2) Å with the p-density of two ethynyl groups of neighboured molecules from both adjacent layers are observed.The interaction energy between molecules within D2 dimer is quite signicant and equal to 45.3 kJ mol −1 .

Fig. 1
Fig. 1 Fragments of crystal structures of 1c and 1c$MeCN showing the formation of (a) centrosymmetric hydrogen-bonded dimer in 1c, (b) molecular layer in 1c$MeCN and (c) columnar motifs (1c, 2c and 1c$MeCN) held by C(p)/B interactions.Note that two different D2 motifs (D2a and D2b) are distinguished in case of 1c and 2c.For 1c$MeCN the symmetry center appears between each pair of molecules within D2 motif.

Fig. 2
Fig. 2 (a) Hirshfeld surfaces generated for 1c molecule with mapped d norm property value over the range −0.50 to 1.30.(b) NBO orbitals participating in the intermolecular interaction with ethynyl group.(c) Combined QTAIM and NCI analysis of 1c.Bond critical points are represented by yellow spheres, BCP corresponding to C(p)/B interaction is represented by blue sphere.The color scale for the NCI plot is −0.01 < sign(l 2 )r < 0.01; Reduced gradient density (RGD) isosurface of 0.3.The green color of RGD surface represents weak attractive interaction.

Table 1
The main structural motifs and interaction energies in crystal structures of 1c, 1c$MeCN and 2c